Lithium ion secondary battery

ABSTRACT

Use of a silicon-based material in a negative electrode of a lithium ion secondary battery results in a decrease in discharge capacity and an increase in internal resistance. In order to overcome this, the lithium ion secondary battery according to the present invention is characterized in having a negative electrode comprising a carbon nanotube having a peak between 2600 and 2800 cm −1  in a Raman spectrum obtained by Raman spectroscopy, a graphite, and a silicon oxide having a composition represented by SiO x  (0&lt;x≤2).

TECHNICAL FIELD

The present invention relates to a lithium ion secondary battery, amethod for manufacturing the same, and a vehicle using a lithium ionsecondary battery.

BACKGROUND ART

Lithium ion secondary batteries are characterized by their small sizeand large capacity and are widely used as power sources for electronicdevices such as mobile phones and notebook computers, and havecontributed to the improvement of the convenience of portable ITdevices. In recent years, attention has also been drawn to the use inlarge-sized applications such as drive power supplies for motorcyclesand automobiles, and storage batteries for smart grids. As the demandfor lithium ion secondary batteries has increased and they are used invarious fields, batteries have been required to have characteristics,such as further higher energy density, lifetime characteristics that canwithstand long-term use, and usability under a wide range of temperatureconditions.

Carbon materials are generally used in a negative electrode of lithiumion secondary batteries. On the other hand, it is also studied thatsilicon materials having a large absorbing and desorbing amount oflithium ions with respect to the unit volume are used in a negativeelectrode for the purpose of high energy density of the batteries.However, the silicon materials deteriorate when charge/discharge oflithium is repeated because they expand and contract. For this reason,they have a problem in cycle characteristics of the batteries.

Various proposals have been made in order to improve the cyclecharacteristics of the lithium ion secondary batteries using the siliconmaterials in negative electrodes. Patent Document 1 discloses batteriescan be improved in rate characteristics and cycle characteristics by anegative electrode comprising (a) a negative electrode active materialsuch as silicon oxide covered with a carbon material, (b) agraphite-based material, and (c) a carbon material other than thegraphite-based materials, such as acetylene black, Ketjen black, powderscontaining graphite crystals, or conductive carbon fiber.

CITATION LIST Patent Document

Patent Document 1: WO2012/140790

SUMMARY OF INVENTION Technical Problem

However, there is a problem in that the decrease in discharge capacityand the increase in internal resistance still have been seen in thelithium ion secondary battery of the above patent document whencharge/discharge cycles are repeated, and further improvement of thecycle characteristics is needed.

An object of the present invention is to provide a lithium ion secondarybattery in which the decrease in discharge capacity and the increase ininternal resistance in using the silicon materials in a negativeelectrode, which are the above problem, are suppressed, and the cyclecharacteristics are improved.

Solution to Problem

The lithium ion secondary battery according to the present inventioncomprises a negative electrode comprising a carbon nanotube having apeak between 2600 and 2800 cm⁻¹ in a Raman spectrum obtained by Ramanspectroscopy, a graphite, and a silicon oxide having a compositionrepresented by SiO_(x) (0<x≤2).

Advantageous Effect of Invention

According to the present invention, a lithium ion secondary batteryhaving more improved cycle characteristics can be provided.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is an exploded perspective view showing a basic structure of afilm package battery.

FIG. 2 is a cross-sectional view schematically showing a cross sectionof the battery of FIG. 1.

FIG. 3 shows Raman spectrums of three types of graphite having differentpeak intensity of D band, G band, and 2D band.

FIG. 4 shows Raman spectrums of three types of silicon oxides havingdifferent peak intensity of D band, G band, and 2D band.

FIG. 5 shows Raman spectrums of three types of carbon nanotubes havingdifferent peak intensity of D band, G band, and 2D band.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described with respect toindividual members of lithium ion secondary batteries.

<Negative Electrode>

The negative electrode has a structure in which the negative electrodeactive material is laminated on a current collector as a negativeelectrode active material layer integrated by a negative electrodebinder. The negative electrode active material is a material capable ofreversibly occluding and releasing lithium ions according to charge anddischarge in a negative electrode.

In the present embodiment, the negative electrode comprises graphite andsilicon oxide as a negative electrode active material and a carbonnanotube as a conductive agent.

The graphite to be used may be either natural graphite or artificialgraphite. Shape of the graphite is not particularly limited and anyshape may be acceptable. Examples of the natural graphite include scalygraphite, flaky graphite, and amorphous graphite, and examples of theartificial graphite include spherical artificial graphite such asmassive artificial graphite and flaky artificial graphite, and MCMB(Mesocarbon microbeads). The graphite to be used may be coated with acarbon material or the like. The median diameter (D_(50G)) of thegraphite particles is preferably within the range of 5.0 μm<D_(50G)<25.0μm. The negative electrode preferably comprises the graphite in anamount of 50% by mass or more, and more preferably 70% by mass or more,based on the total amount of the negative electrode active materialcontained in the negative electrode. In addition, the negative electrodepreferably comprises the graphite in an amount of 97% by mass or lessbased on the total amount of the negative electrode active materialcontained in the negative electrode.

The silicon oxide to be used has a composition represented by SiO_(x)(0<x≤2). An especially preferred silicon oxide is SiO. With respect tothe silicon oxide, it is preferable that the surfaces of the particlesare coated with a carbon material. When the carbon-coated silicon oxideparticles are used, a lithium ion secondary battery excellent in cyclecharacteristics can be provided. The median diameter (D_(50S)) of thesilicon oxide particles is preferably within the range of 0.5μm<D_(50S)<10.0 μm. The negative electrode preferably comprises siliconoxide in an amount of 1% by mass or more, and more preferably 3% by massor more, based on the total amount of the negative electrode activematerial contained in the negative electrode. In addition, the negativeelectrode preferably comprises the silicon oxide in an amount of 20% bymass or less, and more preferably 10% by mass or less, based on thetotal amount of the negative electrode active material contained in thenegative electrode.

Carbon nanotubes are a carbon material formed from planar graphene sheethaving 6 membered rings of carbon, and act as a conductive agent in thesecondary batteries. The carbon nanotubes are formed by making theplanar graphene sheet having 6 membered rings of carbon cylindrical, andmay have a single layer or a coaxial multilayered structure. Both endsof the cylindrical carbon nanotube may be opened and may be closed withhemispherical fullerene containing 5-membered rings or 7-membered ringsof carbon. The diameter of the outermost cylinder of the carbonnanotubes is, for example, preferably 0.5 nm or more and 50 nm or less.The average length (D_(50C)) of the carbon nanotubes is preferablywithin the range of 0.05 μm<D_(50C)<5.0 μm. The negative electrodepreferably comprises the carbon nanotubes in an amount of 0.5% by massor more, and more preferably 1.0% by mass or more, based on the totalamount of the negative electrode active material contained in thenegative electrode. In addition, the negative electrode preferablycomprises the carbon nanotubes in an amount of 20% by mass or less, andmore preferably 5% by mass or less, based on the total amount of thenegative electrode active material contained in the negative electrode.

With respect to the carbon materials having a graphene layer such asgraphite and carbon nanotubes, properties thereof, such as crystallinityand the number of layers, can be confirmed by Raman spectroscopy. In aRaman spectrum obtained by Raman spectroscopy, the peak which occurs inthe range of 2600 to 2800 cm⁻¹ (herein, referred as to “2D band”), thepeak due to in-plane vibration of the graphene which occurs in the rangeof 1500 to 1700 cm⁻¹ (herein, referred as to “G band”), and the peak dueto defects in crystal structure which occurs in the range of 1000 to1400 cm⁻¹ (herein, referred as to “D band”) are commonly used forevaluation of the crystal structure of the graphene layer.

With respect to Raman spectrum of carbon materials, when the carbonmaterial has high peak intensity of G band, it tends to have highcrystallinity, and when the carbon material has high peak intensity of Dband, its crystal fall into disorder and it tends to be structurallydefective. Therefore, the ratio of the peak intensity (I_(G)) of G bandand the peak intensity (I_(D)) of D band has been used as an index ofthe crystallinity, and a large value thereof means that the carbonmaterial has high crystallinity.

2D band also can be used as an index in the same manner. 2D band isknown as an overtone mode of D band. The present inventor found out thatI_(G)/I_(D) has a correlation with the ratio (I_(2D)/I_(D)) of the peakintensity (I_(2D)) of 2D band and the peak intensity (I_(D)) of D bandwhile he investigated Raman spectroscopy of the graphite, siliconoxides, and carbon nanotubes, and the battery properties in detail.I_(G)/I_(D) and I_(2D)/I_(D) relatively show a positive correlation, andwhen I_(G)/I_(D) is large, I_(2D)/I_(D) is also large.

In addition, when the present inventor investigated results of Ramanspectroscopy and battery properties in detail, he found out that 2D banddoes not simply follow D band as its overtone mode, and there are a typefollowing D band sensitively and a type not following D band so much,depending on characteristics of the carbon materials. Examples ofmethods to make the peak intensity of 2D band large regardless of D bandinclude increasing temperature at the time of forming graphite materialsor carbon nanotubes and increasing crystallinity thereof.

It is extremely effective in battery development to investigate carbonmaterials to be used with Raman spectroscopy in detail on the basis ofsuch a trend of the properties and Raman spectrums of the carbonmaterials so as to select lithium ion secondary battery materials. FIGS.3 to 5 show examples of Raman spectrums of the graphite, silicon oxides,and carbon nanotubes, which may be used in the present embodiment.

Carbon nanotubes having 2D band in a Raman spectrum are used in thenegative electrode of the present embodiment. Cycle characteristics ofbatteries can be improved by using the carbon nanotubes having 2D bandin a Raman spectrum in the negative electrode. Although the improvementmechanism of the negative electrode based on presence/absence of 2D bandis not clear in detail, it is considered that low resistance SEI (SolidElectrolyte Interface) film is readily formed on the carbon surface ofthe materials having a peak in 2D band, and, in addition, the materialshaving a peak in 2D band have the effect of improving electrolytesolution retention property, and therefore the cycle characteristics areimproved.

In order to increase the cycle retention ratio and reduce the resistanceincrease rate, it is preferable that the graphite, silicon oxides andcarbon nanotubes contained in the negative electrode exhibit a Ramanspectrum having the peak intensity ratios and/or the peak area ratioswhich will be described later, when they are analyzed by Ramanspectroscopy. The silicon oxide is preferably coated with carbon. Ramanspectrums of the silicon oxide will be described later. In this case,the silicon oxide is coated with carbon, and the Raman spectrums meanthose obtained by Raman spectroscopy of the silicon oxide coated withcarbon. It is considered that the carbon nanotubes showing the peakratios described below tend to form conductive paths between a graphiteparticle and a silicon oxide particle and to suppress destruction of thecarbon coating on the surface of the graphite by the silicon oxide. Inaddition, since the carbon nanotubes exist in the gap between theseparticles, the graphite particles and the silicon oxide particlesshowing the peak ratios described below can follow the expansion andcontraction thereof during charge/discharge. For this reason, the cyclecharacteristics can be improved also by particularly reducing damage ofthe graphite.

When the ratio (I_(G)/I_(D)) of the peak intensity (I_(G)) of G band andthe peak intensity (I_(D)) of D band in a Raman spectrum obtained byRaman spectroscopy is referred to as I_(GG)/I_(GD) with respect to thegraphite, I_(SG)/I_(SD) with respect to the silicon oxide, andI_(CG)/I_(CD) with respect to the carbon nanotube, the peak intensityratios of the graphite, silicon oxide, and carbon nanotube contained inthe negative electrode preferably satisfy at least one of the followingequations, and more preferably all of the following equations.

1<I _(GG) /I _(G) D<20

0.8<I _(SG) /I _(SD)<2

1<I _(CG) /I _(CD)<16

Among the above ranges, I_(GG)/I_(GD) is preferably high, I_(SG)/I_(SD)is preferably close to 1.0, and I_(CG)/I_(CD) is preferably close toI_(SG)/I_(SD). Therefore, the peak intensity ratios of the graphite,silicon oxide, and carbon nanotube contained in the negative electrodepreferably satisfy at least one of the following equations, and morepreferably all of the following equations.

10<I _(GG) /I _(G) D<20

0.9<I _(SG) /I _(SD)<1.2

1<I _(CG) /I _(CD)<2

When the ratio (S_(G)/S_(D)) of the peak area (S_(G)) of G band and thepeak area (S_(D)) of D band in a Raman spectrum obtained by Ramanspectroscopy is referred to as S_(GG)/S_(GD) with respect to thegraphite, S_(SG)/S_(SD) with respect to the silicon oxide, andS_(CG)/S_(CD) with respect to the carbon nanotube, the peak area ratiosof the graphite, silicon oxide, and carbon nanotube contained in thenegative electrode preferably satisfy at least one of the followingequations, and more preferably all of the following equations.

1<S _(GG) /S _(G) D<10

0.8<S _(SG) /S _(SD)<1.2

1<S _(CG) /S _(CD)<10

Among the above ranges, S_(GG)/S_(GD) is preferably high, S_(SG)/S_(SD)is preferably close to 1.0, and S_(CG)/S_(CD) is preferably close toS_(SG)/S_(SD). Therefore, the peak area ratios of the graphite, siliconoxide, and carbon nanotube contained in the negative electrodepreferably satisfy at least one of the following equations, and morepreferably all of the following equations.

4<S _(GG) /S _(GD)<10

0.9<S _(SG) /S _(SD)<1.2

1<S _(CG) /S _(CD)<2

When the ratio (I_(2D)/I_(D)) of the peak intensity (I_(2D)) of 2D bandand the peak intensity (I_(D)) of D band in a Raman spectrum obtained byRaman spectroscopy is referred to as I_(G2)/I_(GD) with respect to thegraphite, I_(S2D)/I_(SD) with respect to the silicon oxide, andI_(C2D)/I_(CD) with respect to the carbon nanotube, the peak intensityratios of the graphite, silicon oxide, and carbon nanotube contained inthe negative electrode preferably satisfy at least one of the followingequations, and more preferably all of the following equations.

0.5<I _(G2D) /I _(GD)<10

0.2<I _(S2D) /I _(SD)<1.0

0.8<I _(C2D) /I _(CD)<7

Among the above ranges, I_(G2D)/I_(GD) is preferably high,I_(S2D)/I_(SD) is preferably close to 1.0, and I_(C2D)/I_(CD) ispreferably close to I_(S2D)/I_(SD). Therefore, the peak intensity ratiosof the graphite, silicon oxide, and carbon nanotube contained in thenegative electrode preferably satisfy at least one of the followingequations, and more preferably all of the following equations.

5<I _(G2D) /I _(G) D<10

0.5<I _(S2D) /I _(SD)<0.9

0.8<I _(C2D) /I _(CD)<1.2

When the ratio (S_(2D)/S_(D)) of the peak area (S_(2D)) of 2D band andthe peak area (S_(D)) of D band in a Raman spectrum obtained by Ramanspectroscopy is referred to as S_(G2D)/S_(GD) with respect to thegraphite, S_(S2D)/S_(SD) with respect to the silicon oxide, andS_(C2D)/S_(CD) with respect to the carbon nanotube, the peak area ratiosof the graphite, silicon oxide, and carbon nanotube contained in thenegative electrode preferably satisfy at least one of the followingequations, and more preferably all of the following equations.

0.5<S _(G2D) /S _(GD)<7

0.2<S _(S2D) /S _(SD)<1.0

0.8<S _(C2D) /S _(CD)<5

Among the above ranges, S_(G2D)/S_(GD) is preferably high,S_(S2D)/S_(SD) is preferably close to 1.0, and S_(S2D)/S_(SD) ispreferably close to S_(S2D)/S_(SD). Therefore, the peak area ratios ofthe graphite, silicon oxide, and carbon nanotube contained in thenegative electrode preferably satisfy at least one of the followingequations, and more preferably all of the following equations.

4<S _(G2D) /S _(GD)<7

0.5<S _(S2D) /S _(SD)<0.9

0.8<S _(S2D) /S _(SD)<1.2

The peak intensity (I_(2D)) of 2D band means the peak intensity of thehighest peak in the range of 2600 to 2800 cm⁻¹. The peak intensity(I_(D)) of D band means the peak intensity of the highest peak in therange of 1000 to 1400 cm⁻¹. The peak intensity (I_(G)) of G band meansthe peak intensity of the highest peak in the range of 1500 to 1700cm⁻¹.

The peak area (S_(2D)) of 2D band means the peak area in the range of2600 to 2800 cm⁻¹. The peak area (S_(D)) of D band means the peak areain the range of 1000 to 1400 cm⁻¹. The peak area (S_(G)) of G band meansthe peak area in the range of 1500 to 1700 cm⁻¹.

In the present embodiment, the cycle characteristics may be furtherimproved by controlling the particle size of the graphite and thesilicon oxide and the length of the carbon nanotube in some cases. It ispreferable that ranges of each median diameter satisfy

5.0 μm<D _(50G)<25.0 μm

0.5 μm<D _(50S)<10.0 μm

0.05 μm<D _(50C)<5.0 μm,

D _(50G) /D _(50S) is 0.5 to 2.0, and

D _(50G) /D _(50C) is 10 to 250,

wherein D_(50G) is a median diameter of the graphite particles, D_(50S)is a median diameter of the silicon oxide particles and D_(50C) is anaverage length of the carbon nanotube. By setting the particle sizes andlength within the above ranges, preferred cycle characteristics can beobtained in some cases. This is presumably because the permeability ofelectrolyte solution is especially improved in the above ranges.

Negative electrode active materials other than the graphite and thesilicon oxide may be additionally used in the negative electrode. Theadditional negative electrode active material is not limited, and knownmaterials may be used. The examples thereof include silicon-basedmaterials such as silicon alloys, silicon composite oxides, and siliconnitride; carbon-based materials such as hardly graphitizable carbon andamorphous carbon; metals such as Al, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd,Pt, Te, Zn, La and alloys thereof; and metal oxides such as aluminumoxide, tin oxide, indium oxide, zinc oxide, and lithium oxide. These canbe used alone or in combination of two or more.

A conductive assisting agent may be further added for the purpose oflowering the impedance. Examples of the additional conductive assistingagent include, flake-like, soot, and fibrous carbon fine particles andthe like, for example, carbon black, acetylene black, Ketchen black,vapor grown carbon fibers and the like.

Examples of the negative electrode binder include polyvinylidenefluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidenefluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene,polypropylene, polyethylene, polyimide, polyamideimide and the like. Inaddition to the above, styrene butadiene rubber (SBR) and the like canbe used. When an aqueous binder such as an SBR emulsion is used, athickener such as carboxymethyl cellulose (CMC) can also be used. Theamount of the negative electrode binder is preferably 0.5 to 20 parts bymass based on 100 parts by mass of the negative electrode activematerial, from the viewpoint of the sufficient binding strength and thehigh energy density being in a trade-off relation with each other. Theabove-mentioned binders for a negative electrode may be mixed and used.

As the negative electrode current collector, from the view point ofelectrochemical stability, aluminum, nickel, copper, silver, and alloysthereof are preferred. As the shape thereof, foil, flat plate, mesh andthe like are exemplified.

The negative electrode may be prepared by forming a negative electrodeactive material layer comprising the negative electrode active materialand the negative electrode binder. Examples of a method for forming thenegative electrode active material layer include a doctor blade method,a die coater method, a CVD method, a sputtering method, and the like. Itis also possible that, after forming the negative electrode activematerial layer in advance, a thin film of aluminum, nickel or an alloythereof may be formed by a method such as vapor deposition, sputteringor the like to obtain a negative electrode current collector.

<Positive Electrode>

The positive electrode includes a positive electrode active materialcapable of reversibly absorbing and desorbing lithium ions with chargeand discharge and it has a structure in which the positive electrodeactive material is laminated on a current collector as a positiveelectrode active material layer integrated by a positive electrodebinder.

The positive electrode active material in the present embodiment is notparticularly limited as long as it is a material capable of absorb anddesorb lithium, but from the viewpoint of high energy density, acompound having high capacity is preferably contained. Examples of thehigh capacity compound include lithium nickel composite oxides in whicha part of the Ni of lithium nickelate (LiNiO₂) is replaced by anothermetal element, and layered lithium nickel composite oxides representedby the following formula (A) are preferred.

Li_(y)Ni_((1-x))M_(x)O₂  (A)

wherein 0≤x<1, 0<y≤1.2, and M is at least one element selected from thegroup consisting of Co, Al, Mn, Fe, Ti, and B.

It is preferred that the content of Ni is high, that is, x is less than0.5, further preferably 0.4 or less in the formula (A). Examples of suchcompounds include Li_(α)Ni_(β)Co_(γ)Mn_(δ)O₂ (0<α≤1.2, β+γ+δ=1, β≥0.7,and γ≤0.2) and Li_(α)Ni_(β)Co_(γ)Al_(δ)O₂ (0<α≤1.2, β+γ+δ=1, β≥0.7, andγ≤0.2) and particularly include LiNi_(β)Co_(γ)Mn_(δ)O₂ (0.75≤β≤0.85,0.05≤γ≤0.15, and 0.10≤δ≤0.20). More specifically, for example,LiNi_(0.8)Co_(0.05)Mn_(0.15)O₂, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂,LiNi_(0.8)Co_(0.15)Al_(0.05)O₂, and LiNi_(0.8)Co_(0.1)Al_(0.1)O₂ may bepreferably used.

From the viewpoint of thermal stability, it is also preferred that thecontent of Ni does not exceed 0.5, that is, x is 0.5 or more in theformula (A). In addition, it is also preferred that particulartransition metals do not exceed half. Examples of such compounds includeLi_(α)Ni_(β)Co_(γ)Mn_(δ)O₂ (0<α≤1.2, β+γ+δ=1, 0.2≤β≤0.5, 0.1≤γ≤0.4, and0.1≤δ≤0.4). More specific examples may includeLiNi_(0.4)Co_(0.3)Mn_(0.3)O₂ (abbreviated as NCM433),LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ (abbreviatedas NCM523), and LiNi_(0.5)Co_(0.3)Mn_(0.2)O₂ (abbreviated as NCM532)(also including those in which the content of each transition metalfluctuates by about 10% in these compounds).

In addition, two or more compounds represented by the formula (A) may bemixed and used, and, for example, it is also preferred that NCM532 orNCM523 and NCM433 are mixed in the range of 9:1 to 1:9 (as a typicalexample, 2:1) and used. Further, by mixing a material in which thecontent of Ni is high (x is 0.4 or less in the formula (A)) and amaterial in which the content of Ni does not exceed 0.5 (x is 0.5 ormore, for example, NCM433), a battery having high capacity and highthermal stability can also be formed.

Examples of the positive electrode active materials other than the aboveinclude lithium manganate having a layered structure or a spinelstructure such as LiMnO₂, Li_(x)Mn₂O₄ (0<x<2), Li₂MnO₃, andLi_(x)Mn_(1.5)Ni_(0.5)O₄ (0<x<2); LiCoO₂ or materials in which a part ofthe transition metal in this material is replaced by other metal(s);materials in which Li is excessive as compared with the stoichiometriccomposition in these lithium transition metal oxides; materials havingolivine structure such as LiFePO₄, and the like. In addition, materialsin which a part of elements in these metal oxides is substituted by Al,Fe, P, Ti, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, La arealso usable. The positive electrode active materials described above maybe used alone or in combination of two or more.

Examples of the positive electrode binder include polyvinylidenefluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidenefluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene,polypropylene, polyethylene, polyimide, polyamideimide and the like. Inaddition to the above, styrene butadiene rubber (SBR) and the like canbe used. When an aqueous binder such as an SBR emulsion is used, athickener such as carboxymethyl cellulose (CMC) can also be used. Amongthem, polyvinylidene fluoride or polytetrafluoroethylene is preferablefrom the viewpoint of versatility and low cost, and polyvinylidenefluoride is more preferable. The above positive electrode binders may bemixed and used. The amount of the positive electrode binder ispreferably 2 to 10 parts by mass based on 100 parts by mass of thepositive electrode active material, from the viewpoint of the bindingstrength and energy density that are in a trade-off relation with eachother.

For the coating layer containing the positive electrode active material,a conductive assisting agent may be added for the purpose of loweringthe impedance. Examples of the conductive assisting agent include,flake-like, soot, and fibrous carbon fine particles and the like, forexample, graphite, carbon black, acetylene black, vapor grown carbonfibers and the like.

As the positive electrode current collector, from the view point ofelectrochemical stability, aluminum, nickel, copper, silver, and alloysthereof are preferred. As the shape thereof, foil, flat plate, mesh andthe like are exemplified. In particular, a current collector usingaluminum, an aluminum alloy, or iron-nickel-chromium-molybdenum basedstainless steel is preferable.

The positive electrode may be prepared by forming a positive activematerial layer comprising the positive electrode active material and thepositive electrode binder. Examples of a method for forming the positiveelectrode active material layer include a doctor blade method, a diecoater method, a CVD method, a sputtering method, and the like. It isalso possible that, after forming the positive electrode active materiallayer in advance, a thin film of aluminum, nickel or an alloy thereofmay be formed by a method such as vapor deposition, sputtering or thelike to obtain a positive electrode current collector.

<Electrolyte Solution>

The electrolyte solution of the lithium ion secondary battery accordingto the present embodiment is not particularly limited, but is preferablya non-aqueous electrolyte solution containing a non-aqueous solvent anda supporting salt which are stable at the operating potential of thebattery.

Examples of the non-aqueous solvent include aprotic organic solvents,for examples, cyclic carbonates such as propylene carbonate (PC),ethylene carbonate (EC) and butylene carbonate (BC); open-chaincarbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC),ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC); aliphaticcarboxylic acid esters such as propylene carbonate derivatives, methylformate, methyl acetate and ethyl propionate; ethers such as diethylether and ethyl propyl ether; phosphoric acid esters such as trimethylphosphate, triethyl phosphate, tripropyl phosphate, trioctyl phosphateand triphenyl phosphate; and fluorinated aprotic organic solventsobtainable by substituting at least a part of the hydrogen atoms ofthese compounds with fluorine atom(s), and the like.

Among them, cyclic or open-chain carbonate(s) such as ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate (BC), dimethylcarbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC),dipropyl carbonate (DPC) and the like is preferably contained.

The non-aqueous solvent may be used alone, or in combination of two ormore.

Examples of the supporting salt include LiPF₆, LiAsF₆, LiAlCl₄, LiClO₄,LiBF₄, LiSbF₆, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiN(CF₃SO₂)₂ and thelike. Supporting salts may be used alone or in combination of two ormore. From the viewpoint of cost reduction, LiPF₆ is preferable.

The electrolyte solution may further contain additives. The additive isnot particularly limited, and examples thereof include halogenatedcyclic carbonates, unsaturated cyclic carbonates, cyclic or open-chaindisulfonic acid esters, and the like. These compounds can improve thebattery characteristics such as cycle characteristics. This ispresumably because these additives decompose during charge/discharge ofthe lithium ion secondary battery to form a film on the surface of theelectrode active material to inhibit decomposition of the electrolytesolution and supporting salt.

<Separator>

The separator may be of any type as long as it suppresses electronconduction between the positive electrode and the negative electrode,does not inhibit the permeation of charged substances, and hasdurability against the electrolyte solution. Specific examples of thematerial include polyolefins such as polypropylene and polyethylene,cellulose, polyethylene terephthalate, polyimide, polyvinylidenefluoride, and aromatic polyamides (aramid) such as polymetaphenyleneisophthalamide, polyparaphenylene terephthalamide andcopolyparaphenylene 3,4′-oxydiphenylene terephthalamide, and the like.These can be used as porous films, woven fabrics, nonwoven fabrics orthe like.

<Secondary Battery>

The lithium ion secondary battery according to the present embodimentmay be, for example, a secondary battery having a structure as shown inFIGS. 1 and 2. This secondary battery comprises a battery element 20, afilm package 10 housing the battery element 20 together with anelectrolyte, and a positive electrode tab 51 and a negative electrodetab 52 (hereinafter these are also simply referred to as “electrodetabs”).

In the battery element 20, a plurality of positive electrodes 30 and aplurality of negative electrodes 40 are alternately stacked withseparators 25 sandwiched therebetween as shown in FIG. 2. In thepositive electrode 30, an electrode material 32 is applied to bothsurfaces of a metal foil 31, and also in the negative electrode 40, anelectrode material 42 is applied to both surfaces of a metal foil 41 inthe same manner. The present invention is not necessarily limited tostacking type batteries and may also be applied to batteries such as awinding type.

As shown in FIGS. 1 and 2, the secondary battery may have an arrangementin which the electrode tabs are drawn out to one side of the outerpackage, but the electrode tab may be drawn out to both sides of theouter package. Although detailed illustration is omitted, the metalfoils of the positive electrodes and the negative electrodes each havean extended portion in part of the outer periphery. The extendedportions of the negative electrode metal foils are brought together intoone and connected to the negative electrode tab 52, and the extendedportions of the positive electrode metal foils are brought together intoone and connected to the positive electrode tab 51 (see FIG. 2). Theportion in which the extended portions are brought together into one inthe stacking direction in this manner is also referred to as a “currentcollecting portion” or the like.

The film package 10 is composed of two films 10-1 and 10-2 in thisexample. The films 10-1 and 10-2 are heat-sealed to each other in theperipheral portion of the battery element 20 and hermetically sealed. InFIG. 1, the positive electrode tab 51 and the negative electrode tab 52are drawn out in the same direction from one short side of the filmpackage 10 hermetically sealed in this manner.

Of course, the electrode tabs may be drawn out from different two sidesrespectively. In addition, regarding the arrangement of the films, inFIG. 1 and FIG. 2, an example in which a cup portion is formed in onefilm 10-1 and a cup portion is not formed in the other film 10-2 isshown, but other than this, an arrangement in which cup portions areformed in both films (not illustrated), an arrangement in which a cupportion is not formed in either film (not illustrated), and the like mayalso be adopted.

<Method for Manufacturing Lithium Ion Secondary Battery>

The lithium ion secondary battery according to the present embodimentcan be manufactured according to conventional method. An example of amethod for manufacturing a lithium ion secondary battery will bedescribed taking a stacked laminate type lithium ion secondary batteryas an example. First, in the dry air or an inert atmosphere, thepositive electrode and the negative electrode are placed to oppose toeach other via a separator to form the above-mentioned electrodeelement. Next, this electrode element is accommodated in an outerpackage (container), an electrolyte solution is injected, and theelectrodes are impregnated with the electrolyte solution. Thereafter,the opening of the outer package is sealed to complete the lithium ionsecondary battery.

<Assembled Battery>

A plurality of the lithium ion secondary batteries according to thepresent embodiment may be combined to form an assembled battery. Theassembled battery may be configured by connecting two or more lithiumion secondary batteries according to the present embodiment in series orin parallel or in combination of both. The connection in series and/orparallel makes it possible to adjust the capacitance and voltage freely.The number of lithium ion secondary batteries included in the assembledbattery can be set appropriately according to the battery capacity andoutput.

<Vehicle>

The lithium ion secondary battery or the assembled battery according tothe present embodiment can be used in vehicles. Vehicles according to anembodiment of the present invention include hybrid vehicles, fuel cellvehicles, electric vehicles (besides four-wheel vehicles (cars, trucks,commercial vehicles such as buses, light automobiles, etc.) two-wheeledvehicle (bike) and tricycle), and the like. The vehicles according tothe present embodiment is not limited to automobiles, it may be avariety of power source of other vehicles, such as a moving body like atrain.

<Power Storage Equipment>

The lithium ion secondary battery or the assembled battery according tothe present embodiment can be used in power storage system. The powerstorage systems according to the present embodiment include, forexample, those which is connected between the commercial power supplyand loads of household appliances and used as a backup power source oran auxiliary power in the event of power outage or the like, or thoseused as a large scale power storage that stabilize power output withlarge time variation supplied by renewable energy, for example, solarpower generation.

EXAMPLE Example 1

(Preparation of Lithium Ion Secondary Battery)

Polyvinylidene fluoride (PVdF) as a binder in an amount of 3 mass %based on the mass of the positive electrode active material, and thelayered lithium nickel composite oxide (LiNi_(0.8)Co_(0.15)Al_(0.05)O₂)in a remaining amount other than the above, are dispersed uniformly inNMP using a rotation revolution type three-axis mixer excellent instirring and mixing, to prepare a positive electrode slurry. Thepositive electrode slurry was uniformly applied to a positive electrodecurrent collector of aluminum foil with a thickness of 20 μm using acoater. After drying by evaporating NMP, the back side was also coatedin the same way. After drying, the density was adjusted by roll press,to prepare positive electrode active material layers on both sides ofthe current collector. Mass per unit area of the positive electrodeactive material layers was 50 mg/cm².

The mixing ratio of artificial graphite, SiO with carbon coating, andcarbon nanotubes in the negative electrode active material was set to93:5:2, and they were dispersed uniformly in 1% by mass aqueous solutionof CMC (carboxymethyl cellulose). Then, a SBR binder (in an amount of 2mass % in the negative electrode) was added there to prepare a negativeelectrode slurry. The negative electrode slurry was uniformly applied toa negative electrode current collector of copper foil with a thicknessof 10 μm using a coater. After drying by evaporating water, the backside was also coated in the same way. After drying, the density wasadjusted by roll press, to prepare negative electrode active materiallayers on both sides of the current collector. Mass per unit area of thenegative electrode active material layers was 20 mg/cm².

Raman spectroscopy was performed on the negative electrode materialswith semiconductor laser having a wavelength of 532 nm. The energydensity was set to 0.1 mW and the measurement was performed with lowlaser intensity which does not damage the samples. The measurement rangeof Raman spectroscopy was 50 to 3500 cm⁻¹. With respect to peakintensity of each material in the profile of Raman spectroscopy, thehighest peak intensity between 1000 cm⁻¹ and 1400 cm⁻¹ was referred toas I_(D), the highest peak intensity between 1500 cm⁻¹ and 1700 cm⁻¹ wasreferred to as I_(G), and the highest peak intensity between 2600 cm⁻¹and 2800 cm⁻¹ was referred to as I_(2D). With respect to peak area, thearea surrounded by a Raman profile and a base line in the range of 1000to 1400 cm⁻¹ was referred to as S_(D), the area surrounded by a Ramanprofile and a base line in the range of 1500 to 1700 cm⁻¹ was referredto as S_(G), the area surrounded by a Raman profile and a base line inthe range of 2600 to 2800 cm⁻¹ was referred to as S_(2D). Ramanspectroscopy of the graphite, silicon oxide, and carbon nanotubes whichwere used as the negative electrode material was performed to calculatethe peak intensity ratios and the peak area ratios respectively.Hereinafter, the peak intensity ratios and the peak area ratios of eachnegative electrode material will be indicated by the abbreviated namesused above.

As an electrolyte solution, 1 mol/L of LiPF₆ as an electrolyte wasdissolved in a solvent of ethylene carbonate (EC):diethyl carbonate(DEC)=30:70 (vol %).

The resulting positive electrode was cut into 13 cm×7 cm, and thenegative electrode was cut into 12 cm×6 cm. The both surfaces of thepositive electrode was covered by a polypropylene separator of 14 cm×8cm, the negative active material layer was disposed thereon so as toface the positive electrode active material layer, to prepare anelectrode stack. Next, the electrode stack was sandwiched by two sheetsof aluminum laminate film of 15 cm×9 cm, the three sides excluding onelong side were heat sealed with a seal width of 8 mm. After injectingthe electrolyte solution, the remaining side was heat sealed, to producea laminate cell type battery.

<Measurement of Capacity Retention Ratio>

300 times of charge-discharge cycle test were performed in athermostatic oven at 45° C. to measure the capacity retention ratio andto evaluate the lifetime. In the charge, the secondary battery wassubjected to constant current charge at 1 C up to maximum voltage of 4.2V and then subjected to constant voltage charge at 4.2 V, and the totalcharge time was 2.5 hours. In the discharge, the secondary battery wassubjected to constant current discharge at 1 C to 2.5 V. The capacityafter the charge-discharge cycle test was measured, and the ratio (%) tothe capacity before the charge-discharge cycle test was calculated. Theresults are shown in Table 1.

<Measurement of Resistance Increase Rate>

The values of electrical resistance (Rsol) were obtained by AC impedancemeasurement. The resistance increase rate of the battery is a valueobtained by dividing the value of electrical resistance (Rsol) after 500times of the charge-discharge cycle test by the value of electricalresistance (Rsol) before the charge-discharge cycle test when the valueof electrical resistance (Rsol) before the charge-discharge cycle testis defined as 1. Small this resistance increase rate means thatresistance components are low, which is preferable because long-lifebattery can be provided.

Examples 2 to 35

Raman spectroscopy was conducted in the same manner as in Example 1. Thegraphite, SiO having carbon coating, and carbon nanotubes, showing peakintensity ratios and peak area ratios of a Raman spectrum shown inTables 1 to 3, were used. Except for that, the batteries were preparedand cycle retention ratios and resistance increase rates were measuredin the same manner as in Example 1.

Comparative Examples 1 to 6

Raman spectroscopy was conducted in the same manner as in Example 1. TheGraphite, SiO having carbon coating, and carbon nanotubes, showing peakintensity ratios and peak area ratios of a Raman spectrum shown inTables 1 to 3, were used. Except for that, the batteries were preparedand cycle retention ratios and resistance increase rates were measuredin the same manner as in Example 1. All of the carbon nanotubes ofComparative examples 1 to 6 did not show the peak of 2D band in a Ramanspectrum, and the peak intensity ratios of 2D band and D band were 0.

Table 1 shows the results of comparing batteries using carbon nanotubesshowing the peak of 2D band in Raman spectrum in the negative electrodeand batteries using carbon nanotubes not showing it in the negativeelectrode. When the carbon nanotubes showing the peak of 2D band wasused, an increase in cycle retention ratio and a decrease in resistanceincrease rate were confirmed and it was demonstrated that the cyclecharacteristics of batteries were improved.

TABLE 1 Cycle Resistance S_(GG)/ I_(SG)/ S_(SG)/ I_(CG)/ S_(CG)/I_(G2D)/ retention increase I_(GG)/I_(GD) S_(GD) I_(SD) S_(SD) I_(CD)S_(CD) I_(GD) S_(G2D)/S_(GD) I_(S2D)/I_(SD) S_(S2D)/S_(SD)I_(C2D)/I_(CD) S_(C2D)/S_(CD) ratio (%) rate Example 1 5 2.5 0.5 0.3 105 3 4 0.3 0.3 1 1.1 80 1.30 Comparative 5 2.5 0.5 0.3 10 5 3 4 0.3 0.3 00 75 1.50 example 1 Example 2 5 2.5 1 0.6 1 0.95 3 4 0.3 0.3 1 1.1 851.24 Comparative 5 2.5 1 0.6 1 0.95 3 4 0.3 0.3 0 0 76 1.51 example 2Example 3 10 5 0.5 0.3 1 0.95 3 4 0.3 0.3 1 1.1 82 1.28 Comparative 10 50.5 0.3 1 0.95 3 4 0.3 0.3 0 0 78 1.52 example 3 Example 4 10 5 1.7 1 105 3 4 0.3 0.3 1 1.1 82 1.28 Comparative 10 5 1.7 1 10 5 3 4 0.3 0.3 0 078 1.52 example 4 Example 5 20 10 1 0.6 1 0.95 3 4 0.3 0.3 1 1.1 88 1.22Example 6 20 10 1 0.6 1 0.95 3 4 0.3 0.3 3.5 1.8 86 1.23 Comparative 2010 1 0.6 1 0.95 3 4 0.3 0.3 0 0 76 1.51 example 5 Example 7 20 10 1.7 110 5 3 4 0.3 0.3 1 1.1 82 1.28 Comparative 20 10 1.7 1 10 5 3 4 0.3 0.30 0 75 1.50 example 6

Table 2 summarizes the results of Examples in which the peak ratios of Gband and D band of graphite, silicon oxide, and carbon nanotubes werechanged.

TABLE 2 Cycle Resistance I_(GG)/ S_(GG)/ I_(SG)/ S_(SG)/ I_(CG)/ S_(CG)/retention increase I_(GD) S_(GD) I_(SD) S_(SD) I_(CD) S_(CD)I_(G2D)/I_(GD) S_(G2D)/S_(GD) I_(S2D)/I_(SD) S_(S2D)/S_(SD)I_(C2D)/I_(CD) S_(C2D)/S_(CD) ratio (%) rate Example 8 5 2.5 0.5 0.3 10.95 3 4 0.3 0.3 1 1.1 80 1.30 Example 9 5 2.5 0.5 0.3 2 1.6 3 4 0.3 0.31 1.1 80 1.30 Example 1 5 2.5 0.5 0.3 10 5 3 4 0.3 0.3 1 1.1 80 1.30Example 2 5 2.5 1 0.6 1 0.95 3 4 0.3 0.3 1 1.1 85 1.24 Example 10 5 2.51 0.6 2 1.6 3 4 0.3 0.3 1 1.1 84 1.25 Example 11 5 2.5 1 0.6 10 5 3 40.3 0.3 1 1.1 83 1.26 Example 12 5 2.5 1.7 1 1 0.95 3 4 0.3 0.3 1 1.1 801.30 Example 13 5 2.5 1.7 1 2 1.6 3 4 0.3 0.3 1 1.1 80 1.30 Example 14 52.5 1.7 1 10 5 3 4 0.3 0.3 1 1.1 80 1.30 Example 3 10 5 0.5 0.3 1 0.95 34 0.3 0.3 1 1.1 82 1.28 Example 15 10 5 0.5 0.3 2 1.6 3 4 0.3 0.3 1 1.182 1.28 Example 16 10 5 0.5 0.3 10 5 3 4 0.3 0.3 1 1.1 82 1.28 Example17 10 5 1 0.6 1 0.95 3 4 0.3 0.3 1 1.1 86 1.23 Example 18 10 5 1 0.6 21.6 3 4 0.3 0.3 1 1.1 85 1.24 Example 19 10 5 1 0.6 10 5 3 4 0.3 0.3 11.1 84 1.25 Example 20 10 5 1.7 1 1 0.95 3 4 0.3 0.3 1 1.1 82 1.28Example 21 10 5 1.7 1 2 1.6 3 4 0.3 0.3 1 1.1 82 1.28 Example 4 10 5 1.71 10 5 3 4 0.3 0.3 1 1.1 82 1.28 Example 22 20 10 0.5 0.3 1 0.95 3 4 0.30.3 1 1.1 84 1.25 Example 23 20 10 0.5 0.3 2 1.6 3 4 0.3 0.3 1 1.1 831.26 Example 24 20 10 0.5 0.3 10 5 3 4 0.3 0.3 1 1.1 82 1.28 Example 520 10 1 0.6 1 0.95 3 4 0.3 0.3 1 1.1 88 1.22 Example 25 20 10 1 0.6 21.6 3 4 0.3 0.3 1 1.1 85 1.24 Example 26 20 10 1 0.6 10 5 3 4 0.3 0.3 11.1 84 1.25 Example 27 20 10 1.7 1 1 0.95 3 4 0.3 0.3 1 1.1 83 1.26Example 28 20 10 1.7 1 2 1.6 3 4 0.3 0.3 1 1.1 83 1.26 Example 7 20 101.7 1 10 5 3 4 0.3 0.3 1 1.1 82 1.28

Table 3 summarizes the results of Examples in which the peak ratios of2D band and D band of graphite, silicon oxide, and carbon nanotubes werechanged.

TABLE 3 Cycle Resistance I_(GG)/ S_(GG)/ I_(SG)/ S_(SG)/ I_(CG)/ S_(CG)/retention increase I_(GD) S_(GD) I_(SD) S_(SD) I_(CD) S_(CD)I_(G2D)/I_(GD) S_(G2D)/S_(GD) I_(S2D)/I_(SD) S_(S2D)/S_(SD)I_(C2D)/I_(CD) S_(C2D)/S_(CD) ratio (%) rate Example 29 20 10 1 0.6 10.95 0.2 0.25 0.1 0.1 0.5 0.3 82 1.28 Example 30 20 10 1 0.6 1 0.95 0.20.25 0.1 0.1 3.5 1.8 85 1.24 Example 31 20 10 1 0.6 1 0.95 0.2 0.25 0.30.3 0.1 0.1 80 1.30 Example 32 20 10 1 0.6 1 0.95 3 4 0.1 0.1 0.1 0.1 801.30 Example 6 20 10 1 0.6 1 0.95 3 4 0.3 0.3 3.5 1.8 86 1.23 Example 520 10 1 0.6 1 0.95 3 4 0.3 0.3 1 1.1 88 1.22 Example 33 20 10 1 0.6 10.95 3 4 0.8 0.8 3.5 1.8 86 1.23 Example 34 20 10 1 0.6 1 0.95 3 10 0.30.3 3.5 1.8 90 1.20 Example 35 20 10 1 0.6 1 0.95 3 10 0.8 0.3 1 1.1 931.18

INDUSTRIAL APPLICABILITY

The battery according to the present invention can be utilized in, forexample, all the industrial fields requiring a power supply and theindustrial fields pertaining to the transportation, storage and supplyof electric energy. Specifically, it can be used in, for example, powersupplies for mobile equipment such as cellular phones and notebookpersonal computers; power supplies for electrically driven vehiclesincluding an electric vehicle, a hybrid vehicle, an electric motorbikeand an electric-assisted bike, and moving/transporting media such astrains, satellites and submarines; backup power supplies for UPSs; andelectricity storage facilities for storing electric power generated byphotovoltaic power generation, wind power generation and the like.

EXPLANATION OF REFERENCE

-   10 film package-   20 battery element-   25 separator-   30 positive electrode-   40 negative electrode

1. A lithium ion secondary battery comprising a negative electrodecomprising a carbon nanotube having a peak between 2600 and 2800 cm⁻¹ ina Raman spectrum obtained by Raman spectroscopy, a graphite, and asilicon oxide having a composition represented by SiO_(x) (0<x≤2). 2.The lithium ion secondary battery according to claim 1, wherein peakintensity ratios of the graphite, the silicon oxide, and the carbonnanotube contained in the negative electrode satisfy the followingequations:1<I _(GG) /I _(GD)<200.8<I _(SG) /I _(SD)<21<I _(CG) /I _(CD)<16 wherein a ratio (I_(G)/I_(D)) of a peak intensity(I_(G)) between 1500 and 1700 cm⁻¹ and a peak intensity (I_(D)) between1000 and 1400 cm⁻¹ in a Raman spectrum obtained by Raman spectroscopy isreferred to as I_(GG)/I_(GD) with respect to the graphite, I_(SG)/I_(SD)with respect to the silicon oxide, and I_(CG)/I_(CD) with respect to thecarbon nanotube.
 3. The lithium ion secondary battery according to claim2, wherein the peak intensity ratios of the graphite, the silicon oxide,and the carbon nanotube satisfy the following equations:10<I _(GG) /I _(GD)<200.9<I _(SG) /I _(SD)<1.21<I _(CG) /I _(CD)<2.
 4. The lithium ion secondary battery according toclaim 1, wherein peak area ratios of the graphite, the silicon oxide,and the carbon nanotube contained in the negative electrode satisfy thefollowing equations:1<S _(GG) /S _(GD)<100.8<S _(SG) /S _(SD)<1.21<S _(CG) /S _(CD)<10 wherein a ratio (S_(G)/S_(D)) of a peak area(S_(G)) between 1500 and 1700 cm⁻¹ and a peak area (S_(D)) between 1000and 1400 cm⁻¹ in a Raman spectrum obtained by Raman spectroscopy isreferred to as S_(GG)/S_(GD) with respect to the graphite, S_(SG)/S_(SD)with respect to the silicon oxide, and S_(CG)/S_(CD) with respect to thecarbon nanotube.
 5. The lithium ion secondary battery according to claim4, wherein the peak area ratios of the graphite, the silicon oxide, andthe carbon nanotube satisfy the following equations:4<S _(GG) /S _(GD)<100.9<S _(SG) /S _(SD)<1.21<S _(CG) /S _(CD)<2.
 6. The lithium ion secondary battery according toclaim 1, wherein peak intensity ratios of the graphite, the siliconoxide, and the carbon nanotube contained in the negative electrodesatisfy at least one of the following equations:0.5<I _(G2D) /I _(GD)<100.2<I _(S2D) /I _(SD)<1.00.8<I _(C2D) /I _(CD)<7 wherein a ratio (I_(2D)/I_(D)) of a peakintensity (I_(2D)) between 2600 and 2800 cm⁻¹ and a peak intensity(I_(D)) between 1000 and 1400 cm⁻¹ in a Raman spectrum obtained by Ramanspectroscopy is referred to as I_(G2D)/I_(GD) with respect to thegraphite, I_(S2D)/I_(SD) with respect to the silicon oxide, andI_(C2D)/I_(CD) with respect to the carbon nanotube.
 7. The lithium ionsecondary battery according to claim 6, wherein the peak intensityratios of the graphite, the silicon oxide, and the carbon nanotubecontained in the negative electrode satisfy the following equations:5<I _(G2D) /I _(GD)<100.5<I _(S2D) /I _(SD)<0.90.8<I _(C2D) /I _(CD)<1.2.
 8. The lithium ion secondary batteryaccording to claim 1, wherein the negative electrode comprises thecarbon nanotube in an amount of 20% by mass or less based on the totalamount of a negative electrode active material.
 9. The lithium ionsecondary battery according to claim 8, wherein the negative electrodecomprises the carbon nanotube in an amount of 5% by mass or less basedon the total amount of a negative electrode active material.
 10. Avehicle equipped with the lithium ion secondary battery according toclaim
 1. 11. A method of producing a lithium ion secondary batterycomprising: a step of stacking a positive electrode and a negativeelectrode via a separator to produce an electrode element and a step ofenclosing the electrode element and an electrolyte solution in an outerpackage, wherein the negative electrode comprises a carbon nanotubehaving a peak between 2600 and 2800 cm⁻¹ in a Raman spectrum obtained byRaman spectroscopy, a graphite, and a silicon oxide having a compositionrepresented by SiO_(x) (0<x≤2).